It has become increasingly important to provide accurate measurements of various parameters such as pressure. One area in which such measurement of pressure is important is the intake manifold pressure which represents an engine fuel demand parameter as shown in the U.S. patent application Ser. No. 881,321 entitled Microprocessor Based Electronic Engine Control System, filed Sept. 27, 1977 and having the same assignee as the subject application.
In the manufacture of a large number of capacitors or, in particular, pressure sensing capacitors or sensors, it is extremely difficult to control all of the parameters with sufficient accuracy to obtain the required capacitance value for each capacitor manufactured. This is particularly true when high precision capacitors are required. As a result, it is either necessary to discard large numbers of capacitors because they do not meet the specified tolerances or to trim the capacitors to value.
The U.S. Pat. No. 3,597,579 to Lumley shows a laser trimming technique for capacitors involving adjusting the effective area of an electrode by applying radiant energy to the electrode. An electrical potential is applied across the electrodes of the capacitor while the radiant energy is applied thereto to inhibit the formation of electrical shorts.
The trimming of a pressure-sensing capacitor is complicated by the fact that the capacitance value of the capacitor varies with pressure. Therefore any trimming of pressure sensing capacitors must allow for reproducible sensor performance through a range of pressures to eliminate or substantially reduce the need for complete calibration of individual sensors.
U.S. Pat. No. 3,750,476 to Brown, issued Aug. 7, 1973 discloses a pressure transducer comprising a pair of sealed members uniformly spaced and coated with an electrically conductive material to define a pair of plates in a capacitor. The U.S. Pat. No. 3,858,097 to Polye issued Dec. 13, 1974 discloses a pressure sensing capacitor comprising a hollow body having spaced opposing walls supported at their edges with electrical conducting means formed on deflecting portions of the wall. Other capacitor pressure transducers are shown in U.S. Pat. Nos. 3,634,727, 3,715,638 and 3,753,373.